As high quality crude oils are depleted, an inexpensive substitute for use as petroleum refinery feedstock becomes more desirable. One such substitute is petroleum residue, or heavy oil, such as that produced from the straight run distillation of crude oil. Another substitute for high quality crude oils are heavier crude oils.
The world's supply of light, sweet crudes has greatly diminished in recent years. Refiners have been forced to deal with ever heavier crudes, containing significantly more metals, while still producing a full spectrum of products. Much of the problem of upgrading these heavier stocks is due to the presence of so much metal, usually nickel and vanadium. The presence of large amounts of metal, usually in association with asphaltenes, presents a formidable upgrading challenge. Some of the worst of these materials are "heavy crudes" while almost as bad are somewhat lighter crudes which contain less asphalt, but even more metal. Each type of resource will be briefly reviewed.